1. Field of the Invention
This invention relates generally to gain flattening, wavelength tunable bandpass filters, and more particularly to gain flattening filters and tunable add/drop filters which do not create unwanted intensity modulation.
2. Description of Related Art
In modern telecommunication systems, many operations with digital signals are performed on an optical layer. For example, digital signals are optically amplified, multiplexed and demultiplexed. In long fiber transmission lines, the amplification function is performed by Erbium Doped Fiber Amplifiers (EDFA""s). The amplifier is able to compensate for power loss related to signal absorption, but it is unable to correct the signal distortion caused by linear dispersion, 4-wave mixing, polarization distortion and other propagation effects, and to get rid of noise accumulation along the transmission line. For these reasons, after the cascade of multiple amplifiers the optical signal has to be regenerated every few hundred kilometers. In practice, the regeneration is performed with electronic repeaters using optical-to-electronic conversion. However to decrease system cost and improve its reliability it is desirable to develop a system and a method of regeneration, or signal refreshing, without optical to electronic conversion. An optical repeater that amplifies and reshapes an input pulse without converting the pulse into the electrical domain is disclosed, for example, in the U.S. Pat. No. 4,971,417, Radiation-Hardened Optical Repeaterxe2x80x9d. The repeater comprises an optical gain device and an optical thresholding material producing the output signal when the intensity of the signal exceeds a threshold. The optical thresholding material such as polydiacetylene thereby performs a pulse shaping function. The nonlinear parameters of polydiacetylene are still under investigation, and its ability to function in an optically thresholding device has to be confirmed.
Another function vital to the telecommunication systems currently performed electronically is signal switching. The switching function is next to be performed on the optical level, especially in the Wavelength Division Multiplexing (WDM) systems. There are two types of optical switches currently under consideration. First, there are wavelength insensitive fiber-to-fiber switches. These switches (mechanical, thermo and electro-optical etc.) are dedicated to redirect the traffic from one optical fiber to another, and will be primarily used for network restoration and reconfiguration. For these purposes, the switching time of about 1 msec (typical for most of these switches) is adequate; however the existing switches do not satisfy the requirements for low cost, reliability and low insertion loss. Second, there are wavelength sensitive switches for WDM systems. In dense WDM systems having a small channel separation, the optical switching is seen as a wavelength sensitive procedure. A small fraction of the traffic carried by specific wavelength should be dropped and added at the intermediate communication node, with the rest of the traffic redirected to different fibers without optical to electronic conversion. This functionality promises significant cost saving in the future networks. Existing wavelength sensitive optical switches are usually bulky, power-consuming and introduce significant loss related to fiber-to-chip mode conversion. Mechanical switches interrupt the traffic stream during the switching time. Acousto-optic tunable filters, made in bulk optic or integrated optic forms, (AOTFs) where the WDM channels are split off by coherent interaction of the acoustic and optical fields though fast, less than about 1 microsecond, are polarization and temperature dependent. Furthermore, the best AOTF consumes several watts of RF power, has spectral resolution about 3 nm between the adjacent channels (which is not adequate for current WDM requirements), and introduces over 5 dB loss because of fiber-to-chip mode conversions.
Dynamic gain flattening filters have been developed that are based on all-fiber acousto-optic tunable filter (AOTF) technology. Acoustic waves traveling along the length of an optical fiber produce periodic microbends that provide strongly wavelength dependent loss in the transmitted optical signals. The center wavelength and the optical attenuation level of the notch filter created can be controlled by the frequency and magnitude of the acoustic waves in the fiber. The wavelength dependent loss mechanism in a single-mode fiber is based on periodic coupling of the guided core mode to non-guiding dissipating cladding modes. By applying multiple acoustic frequencies and properly utilizing multiple cladding modes, one can create quasi-arbitrary filter function that is utilized for flattening the gain of optical amplifiers. This technology provides a number of attractive features including low loss, high speed and low electrical energy consumption. However, it also creates a small amount of unwanted intensity modulation due to the imperfect handling of traveling acoustic waves such as acoustic reflections which are difficult to completely eliminate. Additionally, once the acoustic wave is launched onto the fiber, its properties, such as amplitude, cannot easily be controlled. Thus it is difficult to control the filter spatial perturbation profile which is a function of the acoustic wave amplitude. This limits the flexibility of the dynamic filter.
Accordingly, there is a need for a gain flattening filter which does not create unwanted intensity modulation. There is a further need for a gain flattening filter that allows for the control of its spatial perturbation profile.
Accordingly, an object of the present invention is to provide a new gain flattening, wavelength tunable bandpass filter and an add/drop filter.
Another object of the present invention is to provide gain flattening, wavelength tunable bandpass filters and add/drop filters that are widely tunable.
Still another object of the present invention is to provide a gain flattening, bandpass filter and add/drop filter that has an adjustable transmission.
A further object of the present invention is to provide a gain flattening, bandpass filter and add/drop filter with variable transmission over a fixed wavelength range, and which blocks all other wavelengths.
Yet another object of the present invention is to provide a gain flattening filter that does not create intensity modulation.
A further object of the present invention is to provide a gain flattening filter and add/drop filter that create perturbations in optical fibers to create coherent coupling between two modes
Yet another object of the present invention is to provide a gain flattening filter and add/drop filter that provides polarization independence.
These and other objects of the present invention are achieved in an optical apparatus that include an optical fiber. A plurality of individual optical fiber deformation structures are positioned adjacent along a length of the optical fiber. Each optical fiber deformation structure is configured to introduce a mechanical or index deformation of a portion of the optical fiber to create a perturbation in the optical modes in the fiber. The combined effect of all the perturbations produced by the individual deformation structures is to provide a coherent coupling between two modes in the optical fiber.
In another embodiment of the present invention a dual filter has an optical fiber with an intermediate section. A first filter includes a first plurality of individual optical fiber deformation structures positioned adjacent along a length of the transmission fiber. A second filter has a second plurality of individual transmission fiber deformation structures that are positioned adjacent along a length of the transmission fiber. Each optical fiber deformation structure is configured to introduce a mechanical or index deformation of a portion of the optical fiber to create perturbation in the optical modes in the fiber and provide a coherent coupling between two modes in the optical fiber. The intermediate section is positioned between the first and second filters.
In another embodiment of the present invention an optical apparatus includes an optical circulator with at least a first port configured to be coupled to an input fiber, a second port configured to be coupled to an output fiber and a third port configured to be coupled to an optical fiber. A fiber deformation structure is positioned adjacent along a length of the transmission fiber. The fiber deformation structure includes a plurality of individual optical fiber deformation structures. Each optical fiber deformation structure is configured to introduce a mechanical or index deformation of a portion of the optical fiber to create perturbation in the optical modes in the fiber and provide a coherent coupling between two modes in the optical fiber.